The present invention relates to integrated circuits, and more particularly, to an integrated semiconductor acoustic transducer operating in the ultrasonic range, and to a corresponding fabrication process.
At the present time, acoustic transmitters/receivers operating in the ultrasonic range can not be integrated on a semiconductor chip. This is because an acoustic transmitter/receiver relies on the piezoelectric effect which deforms a thin layer of material due to the effect of an electric field. Furthermore, suitable material is generally a quartz plate which can not be integrated using semiconductor technology.
In view of the foregoing background, an object of the present invention is to provide an integrated semiconductor acoustic transducer.
This and other objects, advantages and features according to the present invention are provided by an integrated semiconductor acoustic transducer comprising, in general, a deformable semiconductor membrane capable of conducting an electric current. The membrane is separated from one zone of a semiconductor substrate by a cavity, thus allowing the membrane to deform.
The substrate is generally made of silicon. Moreover, the membrane is preferably formed from doped silicon so as to allow easier flow of the current therethrough. According to one embodiment, the height of the cavity is on the order of ten nanometers, while the length of the membrane is on the order of a hundred micrometers. These dimensions are particularly suitable for an ultrasound application of the transducer, thus allowing a resonant frequency of about 30 MHz in the case of the membrane, and about 1 MHz in the case of the cavity. This gives the transducer an operating range between 1 and 30 MHz.
According to the invention, the transducer can be used both as an element of an acoustic receiver, and as an element of an acoustic transmitter. When it is used as an acoustic sensor (an element of an acoustic receiver), the transducer advantageously comprises a capacitor having a first plate formed by the semiconductor membrane capable of conducting an electric current and able to deform due to the effect of an acoustic pressure. The pressure variations result from sound propagating through the open air and striking the membrane. The capacitor also includes a second plate formed by a doped zone of the semiconductor substrate and placed opposite the membrane. The cavity separating the two plates then contains, for example, a layer of dielectric gas, such as air, for example.
The invention also provides an acoustic receiver comprising a semiconductor substrate containing at least one transducer as defined above, together with current-generating means capable of generating the current in the membrane of the transducer. The acoustic receiver may further comprise detection means connected to the capacitor for detecting the variations in the capacitance of the capacitor caused by the deformations of the membrane.
When the transducer according to the invention is used as an element of an acoustic transmitter, the semiconductor membrane capable of conducting a modulated electric current is advantageously deformed due to the effect of the Lorenz force. This force is generated by a magnetic field lying in the plane of the membrane, and perpendicular to the lines of current.
The invention also provides an acoustic transmitter comprising a semiconductor substrate including at least one transducer as defined above, together with current-generating means capable of generating a modulated current in the membrane. The acoustic transmitter may further comprise means for generating a magnetic field which is capable of generating the magnetic field lying in the plane of the membrane, and perpendicular to the lines of current. The magnetic field may be generated with a U-shaped magnet, for example.
The invention also provides a process for fabricating an integrated semiconductor acoustic transducer comprising a deformable semiconductor membrane separated from one zone of a semiconductor substrate by a cavity allowing the membrane to deform. The process comprises making, in the substrate, an isolation region defining a semiconductor region called the active region.
The method further comprises depositing, by selective epitaxy on the surface of the active region, a first layer of a first material, for example germanium or a silicon-germanium alloy. The method further comprises depositing, by non-selective epitaxy on the first layer and on the isolating region, a second layer of a second semiconductor material (for example, silicon). The first material is selectively removable with respect to the second material.
The method further comprises locally etching the second layer, the first layer and part of the active region so as to form two lateral trenches which leave a central stack comprising a part of the second layer, a part of the first layer and a part of the active region and which reveal the part of the first layer along two opposed lateral sides of the stack. The part of the first layer is selectively removed from the sides so as to form the cavity which is bounded by the adjacent part of the substrate (forming the zone of the substrate) and the remaining part of the second layer (which forms the membrane). Lateral spacers are formed to close off the cavity beneath the membrane.